Add reference pipeline stage, currently supporting asset-paths only
Allow specifying :debug true to dump expected and contents
Don't reread the less input from disk
Return a pair from build
Add support for data-uri in .ref files
Refactor pipeline.

read-file is now the place to report files not found
assets which compile other assets use build if you want them saved, else use compile
return values of build and compile now include digests
Update README, mention danger
Rename compile and build, and add an extra function, to be less confusing
Fix data-uris
Finish data-uri tests
Fix javascript tests